DESCRIPTION: The proposed research is aimed at understanding the mechanisms involved in the development of a seizure-prone state (epileptogenesis). In preliminary experiments, the important lipid messenger platelet-activating factor (PAF) appears to participate in this process through a receptor-mediated enhancement of excitatory synaptic transmission that leads to a seizure. PAF is known to be produced in brain with seizure, however the importance of this lipid as an intercellular messenger in brain is best evidenced by its role in the Miller-Dieker Lissencephaly syndrome where the overabundance of PAF during neural development results in a smooth cerebral surface and, in the majority of cases, epilepsy. Preliminary studies have shown that PAF augments evoked excitatory synaptic currents and miniature excitatory synaptic currents by a receptor-mediated mechanism. Further studies have shown that in hippocampal slice, PAF may act as part of a retrograde signalling cascade in long term potentiation (LTP). In these studies evidence for PAF acting as a trans-synaptic, synapse specific potentiator is presented. Preliminary studies in hippocampal culture also give evidence for the ability of a PAF-like compound to cross the excitatory synapse in order to produce changes in the presynaptic terminal. Preliminary studies show that preincubation of cultures with pertussis toxin blocks the ability of this PAF-like compound to act in this manner. The experiments proposed are aimed at providing evidence for the ability of PAF and of PAF-like compounds to act as intercellular messengers and produce changes in neighboring neurons that lead to enhanced synaptic efficacy. Based upon preliminary studies and upon the action of PAF in other systems, the signal transduction involved in PAF receptor-mediated enhancement of excitatory synaptic transmission most likely involves a pertussis toxin sensitive G-protein that activates a phosphoinositide specific phospholipase C, releasing intracellular calcium mobilizing second messenger, inositol-1,4,5-triphosphate (IP3). The mobilized calcium enhances excitatory synaptic transmission either directly or through the action of a calcium-dependent enzyme such as calcium calmodulin kinase II which is thought to act upon synapse specific proteins to enhance transmitter release. Experiments are proposed to ascertain the role of these signal transduction mechanisms in the enhanced release of excitatory transmitter by PAF receptor activation.